In some applications it is necessary to protect a faucet or control valve against excessive twisting of the actuating knob or handle. For instance, the faucets at a scrub sink for an operating theater are normally equipped with long handles that can be operated by an elbow or knee. On the one hand the long lever arm afforded by such a structure makes it possible to exert considerable torque on the valve mechanism, and on the other hand a person using an elbow or knee to actuate a mechanism is not able to gauge force accurately.
Thus as described in German patent 2,322,153 of R. Killias it is known to provide a clutch between the handle and the valve spindle. This clutch has a pair of elements pressed axially together by a spring so that the clutch can slip when the valve spindle stops and excessive torque is still being exerted on the handle.
Such an arrangement is relatively effective at protecting the valve, but is fairly complex and bulky. Furthermore it adds considerably to the cost of the valve.